The invention relates to a method for adjusting the color properties or photometric properties of an LED spotlight as well as an apparatus.
Illuminating spotlights having light emitting diodes (LEDs) are known which are used, e.g., as camera attachment light for film and video cameras. Since the LEDs used therefore have either the color temperature “daylight white” or “warm white”, a continuous or exact activation or switch from a warm white to a daylight white color temperature having defined standard color value portions close to or on the Planckian locus is not possible and the color reproduction at film and video recordings is unsatisfactory.
Typical film materials for film recordings like “cinema color negative film” are optimized towards daylight having a color temperature of 5600 K or for incandescent light having a color temperature of 3200 K and achieve extraordinary color reproduction properties for illuminating a set with those light sources. If other artificial light sources are used during film recordings for illuminating a set, they have to be adjusted on the one hand to the optimum color temperature of 3200 K or 5600 K and on the other hand have to have very good color reproduction quality. Regularly, for this purpose the best color reproduction grade having a color rendering index of CRI ≧90 . . . 100 is required.
For an LED spotlight consisting of more than three LED colors, there are unlimited possibilities or possibilities only limited by the resolution of the controlling to adjust a desired chromaticity coordinate color like e.g., x/y=0.423/0.399, CCT=3200 K by mixing the used primary colors. Depending on the mixing ratio, it can be optimized towards different parameters like luminous efficacy or color reproduction. In case of a spotlight primarily used for film and TV recordings, the mixture can additionally be optimized towards the color reproductions properties of the film material or of the sensor of a digital camera. If this optimization is not done, in the most unlikely event the correct chromaticity coordinates x/y are adjusted, but having very unfavorable color reproduction properties. In particular, due to the narrow band spectra of the LED colors like blue, green, red, spectra easily result having an inacceptable color reproduction. Or, however, spectra having good to very good color rendering indices (CRI≧90) which generate at recordings with film or digital cameras significant color deviations as compared to usual light sources like tungsten incandescent or daylight.
It can be deduced from the colorimetry that for such total spectra generated from narrowband LED spectra, optionally also in combination with luminescent material LEDs, never all colorimetric values (chromaticity coordinates, color rendering index as well as mixed-light capability) being relevant for the film and video illumination can adopt ideal values at the same time. Nonetheless, very good results can be achieved if it is guaranteed that none of the optimization parameters deviates too far from the ideal value. However, in the colorimetry no general algorithm is known as to in which ratio more than three spectra have to be mixed to achieve values being as good as possible for the desired chromaticity coordinate, color rendering index as well as mixed-light capability with film at the same time.
However, as in the case of using fluorescent tubes for the illumination of film or video recordings, it can occur in case of artificial light sources having a none-continuous spectral power distribution that these light sources achieve the required values for the color temperature and color rendering index, but nonetheless have a significant color deviation in case of using them for film recordings as compared to tungsten incandescent or HMI lamps or daylight. In this case, one speaks about an insufficient mixed-light capability. This effect can also occur in case of using variously colored LEDs in an LED spotlight. During a test with an LED combination optimized towards a color temperature of 5600K and a color rendering index of CRI=96 at film recordings, a massive red cast as compared to HMI lamps was observed. Also tries with daylight white LEDs did not result in satisfactory results with respect to the mixed-light capability.
US 2004/0105261 A1 discloses a method and an apparatus for emitting and modulating light having a specified light spectrum. The known photometric device has several groups of light emitting apparatuses, each group of which emits a specified light spectrum, and a control device controls the energy supply to the single light emitting apparatuses in such a way that the overall resulting radiation has the specified light spectrum. Thereby, by combination of daylight white and warm white LEDs and modifications of the intensities any color temperatures between the warm white and the daylight white LEDs can be adjusted.
A disadvantage of this method is the also not optimal color reproduction in case of film or video recordings and the lacking possibility to adjust a specified color temperature and an exact chromaticity coordinate. Dependent on the choice of the individual LEDs or the groups of LEDs and the respectively adjusted color temperature, one faces thereby partially significant color deviations from the Planckian locus which can only be corrected by using corrections filters. Additionally, the luminous efficacy is not optimal in case of a warm white setting of the combination of daylight white and warm white LEDs, since hereby relatively high converting losses occur due to the secondary emission of the luminescent material. A further disadvantage of this method is that for adjusting a warm white or daylight white color temperature a main part of the LEDs of the respective other color temperature cannot be used or can only be used highly dimmed so that the utilization factor for the color temperatures around 3200 K or 5600 K typically required in case of film recordings is only approximately 50%.
From DE 20 2005 001 540 U1 a light source for daylight is known which can be adjusted in its color temperature and by which at least one LED emitting white light of a certain color temperature is combined with variously colored light emitting LEDs, in particular in the primary colors red, green and blue. By a modification of the power of single LED colors, a certain color temperature or certain standard light quality can be adjusted by tuning or correcting a specified color temperature or standard light quality automatically by the use of suited sensors, logic and software which can detect the actual spectral power distribution of the light source.
By the use of variously colored LEDs in illuminating spotlights, in particular for photographic or cinematographic recordings, the light of which has a specified color temperature and color rendering index and owns a sufficient mixed-light capability, the following problems occur.
Since LEDs do not emit the emitted light in a monochromatic way with a sharp spectral line but with a band spectrum having certain width so that the emission spectrum of an LED can be assumed as Gaussian bell-shaped curve or as sum of several Gaussian bell-shaped curves and the emission spectra of LEDs can be simulated via the Gaussian distribution. In FIG. 4 some emission spectra of LEDs are exemplarily depicted as function of the relative illumination density over the wavelength, from which can be seen that the wavelength of variously colored light emitting LEDs increases from blue light by green light, amber-colored light towards red light and the form of the emission spectrum of white light emitting LEDs strongly differs from the emission spectra of LEDs emitting differently colored light. This deviation results from the technology of white light generation which is based on the basis of a semiconductor element emitting blue light an being provided with a phosphor covering converting the blue light partially into yellow light resulting in a second, peak in the yellow area of the spectrum besides the first peak in the wavelength area of blue light, a mixed result of which are the portions of white light. Thereby, via the thickness of the phosphor covering, the color temperature can be varied so that in this manner yellowish, warm white as well as daylight white LEDs can be produced.
Additionally, LEDs as illuminant have a strong temperature dependency. With increasing junction temperature, the properties and characteristics of LEDs vary significantly, wherein with increasing temperature the luminance decreases strongly. This is based on the fact that at higher temperature the portion of the radiation-free recombination increases and with increasing temperature a shift of the emission spectra towards higher wavelengths, i.e., towards the red spectrum, is effected. FIG. 5 shows in a schematic depiction the relative luminance over the junction temperature of LEDs which emit blue, green and red light and consist of different material combinations. As a result, the temperature dependency of LEDs is differently strong pronounced in dependence on the used materials what results in the fact that also the colorimetric properties of a light mixture being additively put together from variously colored LEDs vary to achieve a certain color of light or color temperature.
To achieve the color tint or the color temperature of an originally, e.g. at an initial temperature of 20° C., adjusted basic mixture of the light emitted from variously colored LEDs also at a temperature differing from the initial temperature, a spectrometer can be provided and, e.g., be used in the area of the front lens of an illuminating spotlight, which spectrometer measures the spectrum of the light emitted from the illuminating spotlight, or a color sensor is used in the area of the light emitting plane, which color sensor registers deviations of the actual color of the spotlight and then detects the intensity as well as the chromaticity coordinates of the LEDs participating in the light generation in a pulse/measuring mode. Thus, shifts of the peak wavelength as well as variations of the height of the peak wavelength can be detected and, as actual values term, can be fed to a regulation device, the set value of which is the basic setting or basic mixture of the light emitted from the illuminating spotlight. By an according comparison between the set value and the actual value, the light mixture can be corrected to maintain the original spectrum of the basic mixture.
Such a regulation of the color temperature of the light being emitted from an LED spotlight is very complex and time-consuming due to the necessary use of an expensive color sensor and its arrangement in the optical path of the LED spotlight as well as due to the necessary use of a suited computer in connection to a regulation device since in case of such a regulation a temperature-dependent variation of the peak wave length of all LED colors used in the LED spotlight has to be detected and has to be considered during the regulation. The time necessary for this is, e.g., in case of film recordings under different ambient conditions not always available.